1. Field of the Invention
This invention relates to a new process for the production of water-hardenable compositions in the form of mixtures and optionally reaction products containing urea groups, of organic polyisocyanates and ketimine-blocked polyamines containing amino groups, to the compositions obtainable by this process and to their use for the production of water-hardenable lacquers, coating or sealing compositions.
2. Description of the Prior Art
Polyurethane coating compositions of polyisocyanates and blocked polyamines are known. Thus, DE-AS No. 15 20 139 for example describes water-hardenable mixtures of organic polyisocyanates, particularly prepolymers containing terminal isocyanate groups, and polyaldimines or polyketimines derived from polyamines.
Although the Examples of DE-AS No. 15 20 139 use distilled, almost analytically pure polyaldimines or polyketimines as starting materials, some of the mixtures described show only limited stability in storage. This is a practical disadvantage because the mixtures should of course remain liquid and, hence, processible for as long as possible before their application, for example for the formation of surface coverings. The stability in storage of the described mixtures or rather the "pot life" of the coating compositions produced from them is limited by the fact that the described imines, particularly the ketimines, react with isocyanate groups at a measurable velocity, even in the absence of water and at low temperatures. Short pot lives of only a few hours are also a feature of the coating compositions described in U.S. Pat. No. 4,009,307 which contain special isocyanate-terminated polyurethane prepolymers in admixture with special polyketimine materials.
According to DE-AS No. 15 20 139, "sterically hindered" polyaldimines or polyketimines, which show improved stability in storage in admixture with polyisocyanates, are preferably used for avoiding the disadvantages associated with short pot lives. According to DE-AS No. 15 20 139, "sterically hindered" imines are understood to be imines derived from aldehydes or ketones and polyamines in which the carbonyl group and/or at least one amino group is directly attached to a tertiary carbon atom. In addition, the use of aldimines is preferred to the use of ketimines according to the DE-AS in question because, in principle, aldimines show a lesser tendency to react with isocyanates in the absence of water.
However, both the measures proposed in DE-AS No. 15 20 139 for improving the stability in storage of isocyanate-imine mixtures are attended by disadvantages. The use of "sterically hindered" polyaldimines or polyketimines involves considerable expense because these imines cannot be synthesized from the simple polyamines generally used in polyurethane chemistry and/or from standard, inexpensive aldehydes or ketones as starting materials, instead specially synthesized and hence relatively expensive starting compounds have to be used. Another disadvantage of using "sterically hindered" polyaldimines or polyketimines lies in the fact that their reactivity to isocyanates is reduced not only in the absence of water, but also in the presence of water. As comparative tests have shown, coatings based on "sterically hindered" imines of the type in question harden distinctly more slowly than corresponding coatings containing imines of non-"sterically hindered" starting products.
In addition, the preference for aldimines as opposed to ketimines involves disadvantages insofar as aldehydes are released during the moisture hardening of polyisocyanate-polyaldimine mixtures. If they have a low boiling point, as is preferred according to DE-AS No. 15 20 139, aldehydes generally give off a stronger and more offensive odor than the ketones released from ketimines. This particular disadvantage also attends the coating compositions described in U.S. Pat. No. 3,932,357 (DE-OS No. 23 25 824) because these coating compositions are based exclusively on aldimines, preferably isobutyraldimines, which is particularly unfavorable in view of the unpleasant odor of the isobutyraldehyde given off.
Accordingly, the object of the present invention is to provide new, water-hardenable combinations of organic polyisocyanates and at least partly blocked polyamines which show improved stability in storage without any of the disadvantages referred to in the foregoing.
This object is achieved by the development of the process according to the invention which is described in detail hereinafter. The invention is based on the surprising observation that, in the absence of water, polyketimine mixtures which, in addition to "normal" ketimine groups of the structure ##STR3## also contain a certain percentage of groups having the structure ##STR4## react with polyisocyanates as slowly as polyketimines of the type produced from "sterically hindered" synthesis components as defined in DE-AS No. 15 20 139 and that, at the same time, coating compositions containing the polyketimine mixtures suitable for use in accordance with the invention in combination with polyisocyanates harden distinctly more quickly after application to a substrate than corresponding mixtures containing ketimines of the "sterically hindered" synthesis components under otherwise comparable conditions.
The particular significance of the invention is attributable to the fact that the polyketimine mixtures suitable for use in accordance with the invention are directly obtainable from standard, non-sterically hindered polyamines and simple, non-sterically hindered ketones. This is because, by maintaining certain reaction conditions, the condensation reactions leading to Schiff's bases can be displaced in such a way that products "condensed to a relatively high degree" in the context of aldol condensation and containing functional groups of the structure ##STR5## are formed. Reactions of this type are known, for example from A. M. Paquin, Chem. Ber. 82, 316 (1949) and U.S. Pat. No. 2,498,419. The polyamines and ketones suitable for use in accordance with the present invention may be reacted with one another in such a way as to form polyketimine mixtures containing functional groups of the structure ##STR6## in addition to groups of the structure ##STR7## in such quantitative ratios that the crude product formed may be directly combined with polyisocyanates in accordance with the invention, optionally after the removal of solvents.